Mechanism of reactions of unsaturated ethers and thioethers XII: Reaction of 1-ethoxyvinyl esters with carboxylic acids in non-aqueous solvents†

The reaction of 1-ethoxyvinyl esters and carboxylic acids has been investigated in non-aqueous solvents. The kinetics in benzene follow the rate expression V = k30 · C1-ethoxyvinyl ester · C2acid while in dioxan and sulpholane the following equation is obeyed V = (k30 · C2acid + k4 · C3acid) · C1-ethoxyvinyl ester The rate constants in sulpholane are 20 to 30 times as large as those in dioxan, while the rate of reaction in benzene is approximately equal to that in sulpholane. The kinetic isotope effect, kH/kD, for the reaction of 1-ethoxyvinyl acetate and either acetic acid or O-deuteroacetic acid, amounts to 4.0 in benzene and to 3.6 in sulpholane as solvent. During this reaction no deuterium was incorporated into the vinyl ester molecule. Addition of tetrabutylammonium perchlorate to the solvent dioxan increases the rate constant of the reaction between 1-ethoxyvinyl chloroacetate and chloroacetic acid in accordance with the equation k3 = (k3)conc. salt = O (1 + b · Csalt) The value of the coefficient b amounts to 4 1/mole. From these results it was concluded, that in a number of pre-equilibria a solvation complex is formed between a molecule of 1-ethoxyvinyl ester and two, three or four molecules of carboxylic acid. These pre-equilibria are followed by the rate determining step in which a proton is transferred to the substrate giving an ion pair as intermediate. The products are formed from this ion pair.